1. Field of the Invention
The present invention relates to a power transmission system for four-wheel drive vehicles which distributes a proportion of the torque of the main driven wheels which are directly driven by an engine to auxiliary driven wheels via a multiple disk clutch.
2. Description of the Related Art
Such a power transmission system for four-wheel drive vehicles is disclosed in FIG. 10 of Japanese Patent Application Laid-open No. 9-202152. In this system, a driving shaft which rotates in operative connection with front wheels which are the main driven wheels and a driven shaft which rotates in operative connection with rear wheels which are the auxiliary driven wheels are connected to each other via a multiple disk clutch, and a bidirectional clutch mechanism is provided on the aforementioned driven shaft. The bidirectional clutch mechanism has the function of enhancing the ground covering properties of the vehicle by being engaged when the front wheels slip, which causes the rotational rate of the front wheels to exceed the rotational rate of the rear wheels, thereby distributing the torque of the front wheels to the rear wheels when the vehicle is travelling either forward or backward, and the function of avoiding influencing the operation of the ABS (anti-lock braking system) by cancelling the engagement when the front wheels are locked, which causes the rotational rate of the front wheels to become lower than the rotational rate of the rear wheels, so as to prevent the torque of the front wheels from being distributed to the rear wheels.
In the above-mentioned conventional system, because the bidirectional clutch mechanism is provided on the driven shaft which transmits the torque of the front wheels to the rear wheels, the aforementioned torque is transmitted directly via the bidirectional clutch mechanism. Therefore, it is necessary to use a large and expensive bidirectional clutch mechanism having a large torque transmission capacity, which is the main cause of the increase in size and cost of the power transmission system.
The present invention has been conceived in view of the above-mentioned circumstances, and it is an object of the present invention to reduce the torque transmission capacity of a bidirectional clutch mechanism which is used in a power transmission system for four-wheel drive vehicles and to reduce the size and cost of the bidirectional clutch mechanism.
In accordance with a first aspect of the present invention, in order to achieve the above-mentioned object, a power transmission system for four-wheel drive vehicles is proposed for distributing a proportion of the torque of the main driven wheels which are directly driven by an engine to auxiliary driven wheels via a driving shaft, a multiple disk clutch and a driven shaft, comprising a torque cam mechanism which comprises a first cam element and a second cam element which can rotate relative to each other and which generates a thrust force for engaging the multiple disk clutch by the relative rotation of the two cam elements, a bidirectional clutch mechanism which comprises a first clutch element and a second clutch element which can rotate relative to each other and which engages the two clutch elements with each other regardless of the rotational direction of the first clutch element when the rotational rate of the first clutch element exceeds the rotational rate of the second clutch element and a load generating means comprising a first rotor and a second rotor which can rotate relative to each other which generates a rotational load by the relative rotation of the two rotors, wherein the driving shaft is connected to the first cam element of the torque cam mechanism, the second cam element of the torque cam mechanism is connected to the first rotor of the load generating means, the second rotor of the load generating means is connected to the first clutch element of the bidirectional clutch mechanism, and the second clutch element of the bidirectional clutch mechanism is connected to the driven shaft.
In accordance with a second aspect of the present invention, in addition to the above-mentioned first aspect, a power transmission system for four-wheel drive vehicles is proposed in which the above-mentioned load generating means is a hydraulic pump.
In accordance with a third aspect of the present invention, in addition to the above-mentioned first aspect, a power transmission system for four-wheel drive vehicles is proposed in which the above-mentioned load generating means is a power generator.
In accordance with the above-mentioned arrangements, the bidirectional clutch mechanism is in a disengaged state when the vehicle is travelling forward at a constant speed, where the rotational rate of the main driven wheels coincides with the rotational rate of the auxiliary driven wheels and when the vehicle is braking when travelling forward where the rotational rate of the main driven wheels is less than the rotational rate of the auxiliary driven wheels. As a result, the second rotor of the load generating means rotates under no load by being dragged by the first rotor, the torque cam mechanism does not transmit any torque and no thrust force is thus generated, the multiple disk clutch is disengaged, and the vehicle is maintained in a two-wheel drive state.
Because the bidirectional clutch mechanism is in an engaged state when the vehicle starts to travel forward and when the vehicle accelerates in the forward direction where the rotational rate of the main driven wheels exceeds the rotational rate of the auxiliary driven wheels, the first clutch element of the bidirectional clutch mechanism brakes the second rotor of the load generating means so causing rotation relative to the first rotor. As a result, the load generating means generates a load, the torque cam mechanism transmits the torque so as to generate a thrust force, the multiple disk clutch is therefore engaged and the vehicle switches over to a four-wheel drive state.
When the vehicle is travelling backward the direction in which each of the elements of the power transmission system rotates is opposite to the rotational direction when the vehicle is travelling forward, and since the bidirectional clutch mechanism engages the first clutch element with the second clutch element regardless of the rotational direction of the first clutch element when the rotational rate of the first clutch element exceeds the rotational rate of the second clutch element, the bidirectional clutch mechanism is disengaged when the vehicle is travelling backward at a constant speed and when the vehicle is being braked backward in the same manner as when it is travelling forward so as to maintain the vehicle in a two-wheel drive state, and the bidirectional clutch mechanism is engaged so as to switch the vehicle over to a four-wheel drive state when the vehicle starts to travel backward and when the vehicle accelerates backward.
Torque transmitted from the main driven wheels to the auxiliary driven wheels is not directly applied to the bidirectional clutch mechanism; only a small torque which is transmitted by the torque cam mechanism is applied to the bidirectional clutch mechanism, and it is therefore possible to decrease the torque transmission capacity of the bidirectional clutch mechanism, thereby reducing the size and the cost thereof.
With regard to the load generating means, a hydraulic pump or a power generator can be used.
The above-mentioned objects, other objects, characteristics and advantages of the present invention will become apparent from an explanation of preferable embodiments which will be described in detail below by reference to the attached drawings.